Mike,
Jim Carter has a technical article on "The Forgotten 261". His site is set up so that it is not possible to directly link to the Technical Articles. Find the link on the right side of his homepage.
http://www.oldchevytrucks.com/cgi/htmlos.exe/002893.4.685387737600026637 1954 to early 1955 were the years that had "captain bars"on the 235 that were similar to those on 261s.
1958-1962 had the 1/2 NPT fitting for a full-flow oil filtration system option.
I think you will need more information before you can assure yourself that it is a 1954 261. Jim Carter's article has good information on head/block casting numbers.
Tim
